This invention relates to an improved tape initialization process, and, more particularly, to staring in memory tape/drive calibration and initialization information for quick access to data stored on a tape.
Magnetic tape data storage typically provides prerecorded servo tracks to allow precise positioning of a tape head which has servo sensors, with respect to the prerecorded servo tracks. The tape head comprises one or more read/write elements precisely positioned with respect to the servo sensors and which trace data tracks parallel to the servo tracks. One example of a magnetic tape system is the IBM 3590, which employs magnetic tape having prerecorded servo patterns that include three parallel sets of servo edges, each servo edge being an interface between two dissimilar recorded servo signals, each set of servo edges comprising one servo edge on each of opposite lateral sides of a middle recorded servo signal.
The tape head has several spaced apart servo sensors for each servo edge, with the result that the tape head may be stepped between the servo sensors, each positioning the read/write elements at different interleaved groups of data tracks.
Typically, for a given servo pattern of a set of two servo edges, the outer servo signals are recorded first, and the center servo signal is recorded last, to provide the servo edges. As pointed out, by the incorporated ""159 patent, the nominal separation distance between the servo edges of each set of servo edges is a certain distance, such as 80 microns, but there is variation in the magnetic separation between the servo edges, for example, due to the variation of the width of the physical write element which prerecords the servo pattern, due to variation in the magnetic characteristics of the physical write element, etc. The variation may occur between servo tracks in a single magnetic tape, and may occur between prerecording devices and therefore between magnetic tapes.
To reduce the apparent difference of the edge separation distance of the prerecorded servo tracks from nominal, the prerecording of the servo tracks is conducted at different amplitudes so as to attempt to compensate for the physical difference and provide a magnetic pattern that is closer to nominal. Additionally, three servo sensors are employed to simultaneously sense the three servo tracks, and, the average of the servo signals may be employed to track follow the servo tracks. Thus, the difference in physical distance and in amplitude compensation may tend to offset as between the servo tracks. These actions may provide an adequate signal for track following at the servo edges.
However, to increase track density, the servo sensors may themselves be indexed to positions laterally offset from the linear servo edges to provide further interleaved groups of data tracks. The indexed positions are determined by measuring the ratio between the amplitudes of the two dissimilar recorded servo signals. Thus, when the amplitudes of the recorded servo signals are varied to compensate for physical distance variations, track following the prerecorded servo edges at the offset indexed positions becomes less precise. As the result, the data tracks may vary from the desired positions, for example, squeezed together, such that writing on one track with a write element that is subject to track misregistration (TMR) may cause a data error on the immediately adjacent data track.
The tape uses an analog Position Error Signal (PES) that is written on the tape in one or more dedicated servo position areas. These areas are used to correctly position read/write heads over the data portion of a tape using servo heads to read the servo position areas. This servo signal is written 3 times across the tape, and as will be described herein below, is used as the positioning mechanism for the tape drive system. In order to ensure that a given tape/tape drive system are operating correctly, a calibration sequence is performed each time the tape is inserted into the drive mechanism. As the data track density increases, this initialization and calibration sequence becomes ever more critical, since even slight mismatches may result in the destruction of entire tracks of data. This lengthy process, as will be described below, involves the use of an independent position sensor (e.g., optical sensor).
The calibration process must essentially account for three error sources associated with tape and tape drive mechanisms. The first is the servo head. The non-uniformity of the servo readers requires a measurement and calibration of the offsets associated with a particular head. The next source of error is the tape. The servo patterns written on the tape must be written so that the servo edges appear to be some preset distance apart. For example, when using 40 micron data tracks the servo edges should be written to appear 80 microns apart when read by the servo heads. The servo track writer uses a three module head, but the write module that determines the width of the pattern is less than 80 microns wide. To make the servo pattern appear to be 80 microns wide that write currents in the outer servo elements are adjusted until the patterns appear to be 80 microns wide when read. Another source of error is interaction between the individual servo elements with a written pattern. Each reader should be calibrated on both servo edged at multiple positions to enable sufficient accuracy.